The human retinoid X receptors (hRXRs) consist of three identified isoforms (α, β, γ) that function as transcription promoters often in partnership with other members of a larger nuclear receptor (NR) family of transcription regulators including the thyroid receptor (TR), the vitamin D receptor (VDR), the liver X receptor (LXR), the peroxisome proliferator-activated receptor (PPAR), and the retinoic acid receptor (RAR). While 9-cis-retinoic acid (9-cis-RA) and docosahexaenoic acid (DHA) have been shown to bind to hRXRs and promote RXR element (RXRE) regulated transcription (i.e. function as RXR agonists), it is still unclear if RXR has a bona fide endogenous molecular ligand. RXR has been described as the central NR regulator, because it often plays a critical role, either as a permissive or non-permissive partner, in heterodimer complexes that must be formed with the other NRs to regulate their respective response elements.
Recent studies have identified several RXR-selective-binding molecular ligands (rexinoids) that can modulate not only RXRE regulated transcription but also the heterodimer regulated transcription of other NRs. For instance, RXR is a subordinate partner in the RXR-RAR heterodimer, otherwise referred to as a non-permissive heterodimer, since transcription is not promoted in the RAR unliganded (apo-RAR) heterodimer with RXR. Additionally, the RXR-TR heterodimer is non-permissive. In contrast to these non-permissive heterodimers, permissive heterodimers such as RXR-PPAR allow transcription to be promoted in the presence of either RXR or PPAR agonists. The RXR-LXR heterodimer is also permissive. Hence, there is enormous potential for RXR agonists to activate or repress various biological pathways and effect therapeutic results for various conditions that would benefit from activation or repression of a specific pathway.
Six rexinoids described in the literature include Bexarotene (60), CD3254 (61), LGD100268 (62), a pyridyl-bexarotene analog (1), an unsaturated bexarotene analog (2), and the mono-fluorinated bexarotene analog (3).
Bexarotene has been documented to have an EC50 of 33, 24 and 25 nm for the RXR α,β,γ subtypes, respectively, and a Kd of 14, 21, and 29 nm for the RXR α,β,γ subtypes, respectively, in a CV-1 cell line (Boehm, M. F., et al., “Synthesis and Structure-Activity Relationships of Novel Retinoid X Receptor-Selective Retinoids” J. Med. Chem. 1994, 37, 2930-2941). CD3254 appears to have an EC50 on the order of 10 nm for the hRXRβ isoform (Santin, E. P., et al., “Modulating Retinoid X Receptor with a Series of (E)-3-[4-Hydroxy-3-(3-alkoxy-5,5,8,8-tetramethyl-5,6,7,8-tetrahydronaphthalen-2-yl)phenyl]acrylic Acids and Their 4-Alkoxy Isomers” J. Med. Chem. 2009, 52, 3150-3158). LGD100268 and 1 have been documented to have EC50s of 4, 3, and 4 nm and 6, 9, and 5 nm for the RXR α,β,γ subtypes, respectively, and Kds of 3, 3, and 3 nm and 22, 61, and 39 nm for the RXR α,β,γ subtypes, respectively, in a CV-1 cell line (Boehm, M. F., et al., “Design and Synthesis of Potent Retinoid X Receptor Selective Ligands That Induce Apoptosis in Leukemia Cells” J. Med. Chem. 1995, 38, 3146-3155). While the unsaturated-bexarotene analog (2) has been reported, its ability to serve as an RXR agonist has not been published. Finally, the mono-fluorinated bexarotene analog (3) has an EC50 of 43 nm and a Kd of 12 nm in hRXR in Caco-2 cells (Wagner, C. E., et al., “Modeling, Synthesis and Biological Evaluation of Potential Retinoid X Receptor (RXR) Selective Agonists: Novel Analogues of 4-[1-(3,5,5,8,8-Pentamethyl-5,6,7,8-tetrahydro-2-naphthyl)ethynyl]benzoic Acid (Bexarotene)” J. Med. Chem. 2009, 52, 5950-5966).
Currently there is a need for additional chemical agents that are useful for treating or preventing cancer or treating or preventing Alzheimer's disease. There is also a need for anti-cancer or anti-Alzheimer's agents that have enhanced activity or that have improved pharmacologic properties such as increased solubility or better bioavailability.
Additionally, studies suggest that the retinoid X receptor pathway is associated with CNS remyelination processes (see M. Natrajan, et al., Brain, 2015, 1-17; and J. K. Huang et al., Nature Neuroscience, 2010, 1). Currently there is a need for additional chemical agents that are useful for treating conditions associated with demyelination, such as, for example, multiple sclerosis.